Thomas Aquinas is a well-known theorist of the natural law who also coincides, in period, with the typical high mediaeval setting of D&D. And here is Aquinas on theft (I believe I'm quoting from Summa Theologica, II-II, Question 66, Article 7):
[M]aterial goods are provided for the satisfaction of human needs. Therefore the division and appropriation of property, which proceeds from human law, must not hinder the satisfaction of man’s necessity from such goods. Equally, whatever a man has in superabundance is owed, of natural right, to the poor for their sustenance . . .
But because there are many in necessity, and they cannot all be helped from the same source, it is left to the initiative of individuals to make provision from their own wealth, for the assistance of those in need. If, however, there is such urgent and evident necessity that there is clearly an immediate need of necessary sustenance – if, for example, a person is in immediate danger of physical privation, and there is no other way of satisfying his need – then he may take what is necessary from another person’s goods, either openly or by stealth. Nor is this, strictly speaking, fraud or robbery.
My impression is that many D&D players would equate that outlook with CG rather than LG.
I'm actually a philosopher, and that may be why I've always found Planescape and its factions unbearably silly.